Since the latter seems a valid option for you let me explain in short what you'll need and how it works.

You'll need:

- A working NIM server at the same or higher level than the target client level
- The filesets bos.alt_disk_install.boot_images, bos.alt_disk_install.rte installed on the NIM server.
- A complete lppsource NIM resource at the level you're going to migrate the client to, which means including all TLs and SPs needed for that level.
- The above filesets bos.alt_disk_install.boot_images, bos.alt_disk_install.rte contained in this lppsource.
- A SPOT created from the mentioned lppsource.
- rsh communications between server and client: rshd enabled in /etc/inetd.conf and started on the client, the NIM server's hostname contained in ~root/.rhosts of the client.
- A free hdisk on the client at least as big as the client's original rootvg disk. This disk must not be a member of any volume group! If the client's rootvg is mirrored a migration can be done to a single target disk nevertheless.

How nimadm works behind the stage:

- Create a copy of the client's rootvg named altinst_rootvg with LVs named altinst_<lvname>.
- Export these LVs and mount them on the NIM server via NFS
- Migrate mounted LVs to new level on the NIM server by means of an lppsource and a SPOT at the target level.
- Change the client's bootlist to boot from the new disk next time.
- Rename altinst_rootvg to rootvg, rename rootvg to old_rootvg, rename LVs according to the same scheme at the reboot.

What you must do:

Having created the lppsource and the SPOT, having established rsh communication and having provided a free disk just run on the NIM server:

nimadm -c client_name -s spot_name -l lppsource_name -d hdiskname -Y

If something goes wrong:

1) If you already tried to reboot the client:
Set the client's bootlist to boot from the old disk and reboot. Use the SMS menus for this if required.
Run nimadm -C -c client_name -s spot_name on the server to clean everything up.
Correct errors and try again.